Mechanism of thermodormancy induction in Fraxinus mandshurica seeds is related to changes in the endosperm

Does thermodormancy impact the embryo and/or the endosperm? The seed-embryo replacement method was used to determine the mechanism of thermodormancy induction in the endospermic seeds of Fraxinus mandshurica. Germination of "new seeds" after seed embryo replacement (thermodormant embryo + non-dormant endosperm, non-dormant embryo + thermodormant endosperm, thermodormant embryo + thermodormant endosperm, non-dormant embryo + non-dormant endosperm) was compared. Germination of isolated embryos in exogenous hormones and endosperm extracts, endosperm cell wall-degrading enzyme activity, and endosperm hormone content were measured. Endosperm transcriptome of non-dormant and thermodormant seeds was determined using RNA-Seq sequencing technology. The embryos of non-dormant seeds and thermodormant seeds were not dormant, and germination of embryos isolated from them was the same. The twofold dilution of endosperm extract significantly inhibited embryos growth of non-dormant and thermodormant seeds, while the germination percentage (GP) was lower than 15%. Regardless of whether the embryo of the "new seed" came from a thermodormant or non-dormant seed, the GP of the "new seed" was higher if the endosperm came from a non-dormant seed (80 and 84%, respectively). However, if the endosperm came from a thermodormant seed, GP of the new seed with an embryo from a thermodormant or non-dormant seed decreased significantly (64 and 66%, respectively). The activity of cell wall-degrading enzymes in radicle-end endosperm of thermodormant seeds was lower than that in non-radicle-end endosperm, and the activity of enzyme in radicle-end endosperm of seeds decreased significantly after cultivating for more than 5 days at high temperature (HT). ABA content in endosperm increased significantly, GA₃ content in endosperm decreased significantly, and GA₃/ABA ratio of endosperm was significantly decreased by nearly 1/3. HT triggers stress response by activating ABA biosynthesis and the corresponding signaling pathways. Therefore, the embryo of thermodormant F. mandshurica seeds was non-dormant, and thermodormancy induction was related to changes in the endosperm. During incubation at high temperature, the softening ability of endosperm (especially in radicle-end endosperm) was significantly weakened, while ABA accumulation and GA₃ decomposition in endosperm significantly enhanced the inhibition of germination by endosperm. High temperature strongly activated ABA-related signaling pathways and stress response mechanisms in endosperm. MAIN CONCLUSIONS: Induction of F. mandshurica seeds into thermodormancy is related to changes in the endosperm. The embryo of the thermodormant seeds F. mandshurica is non-dormant.